This invention relates generally to photoactive semiconductor film electrodes suitable for use in electrochemical cells and in "photoassisted" electrochemical reactions utilizing true solid/solid solutions of diverse mixed metal oxides.
There has been considerable recent interest in the application of the photoactive semiconductor film electrodes to the electrolysis of water and to the direct conversion of solar energy to electrical, fuels, chemicals and/or chemical energy. The uses of such electrodes have been generalized as reduction-oxidation reactions in addition to the electrolysis of water. Oxidation reactions at n-type photoactive semiconductor electrodes and reduction reactions at p-type photoactive semiconductor electrodes can be carried out at potentials lower than ordinarily required utilizing light as an additional driving force for these reactions. Such processes have been termed "photoassisted" rather than photocatalyzed reactions. There are, however, two major obstacles which must be overcome in order to make direct conversion of solar energy to electrical, fuels, chemicals and/or chemical energy and/or the electrolysis of water a viable commercial process both on the industrial level and on the consumer level. The first of these two problems is reducing the cost of producing the desired end result, i.e., electrolysis of water or direct conversion of solar energy to electrical, fuels, chemicals and/or chemical energy. The second problem is producing a system that has a long life in actual use. An acceptable life span is generally thought to be twenty years. Oxides of aluminum, niobium, tantalum, titanium and tin, for example, answer both of the above problems and do exhibit photochemical response. It has been long known, for example, that titanium dioxide (TiO.sub.2) fills both of the requirements of long life and economical production of n-type and p-type electrodes for use in the electrolysis of water or the direct conversion of solar energy to chemical or electrical energy. However, titanium dioxide by itself has an unacceptably large "band gap" in relation to the solar spectrum. The term "band gap" as herein and hereafter used means the amount of energy (measured, for example, as eV) needed to raise an electron in a valence band to the lowest available energy level conduction band. This band gap is too wide for use with 97 percent of the available solar energy spectrum for pure TiO.sub.2, i.e., TiO.sub.2 absorbs wavelengths that are shorter than 400 nanometers and 97 percent of the terrestrial solar spectrum has wavelengths that are longer than 400 nanometers. Titanium dioxide does have an additional advantage of being a material which is not toxic to the general environment. Thus it does not have any of the generally harmful effects, to the environment, commonly associated with materials having a band gap more closely attuned to the major energy output portion of the solar spectrum such as, for example, compounds formed from such elements as selenium, gallium, cadmium, tellerium and/or arsenic.
It is known that electrodes fabricated from, for example, single crystals of pure titanium dioxide, doped single crystals of titanium dioxide, or polycrystalline titanium dioxide, which may or may not be deposited on an appropriate substrate, can be used as photoelectrodes. Titanium dioxide has a band gap which is unacceptably high, i.e., about 3.0 eV. This band gap results in a maximum terrestrial power conversion efficiency of only about 1 or 2 percent. To form electrically conductive, semiconductor material, the titanium dioxide is typically treated by reduction with hydrogen or reduction in a vacuum. It is theorized that such treatment produces a material with oxygen lattice deficiencies in the titanium dioxide crystal, with these lattice defect sites contributing to the semiconductor properties. This partially reduced material can be characterized by the general formula TiO.sub.(2-x), where x takes on a value of between 0 and 1. Because of the great possibilities which these electrodes have for conversion of solar energy to electricity, fuels, chemicals and/or chemical energy, a number of studies have been directed to methods of fabricating electrodes which make such conversions more efficient. In previously described uses of n-type titanium dioxide semiconductor electrodes, it has generally been the practice to use electrodes formed from single crystals of TiO.sub.2 or a polycrystalline TiO.sub.2.
The technique of producing single crystal, photoactive TiO.sub.2 electrodes is described, for example, by S. N. Frank et al. in "Semiconductor Electrodes 11: Electrochemistry at N-type TiO.sub.2 Electrodes in Acetonitrile Solutions,"J. Am. Chem. Soc. Vol. 97:7427 (1975). Polycrystalline titanium dioxide electrodes produced by chemical vapor deposition techniques are described, for example, by K. L. Hardee et al. in "The Chemical Vapor Deposition and Application of Polycrystalline in N-type Titanium Dioxide Electrodes to the Photosensitized Electrolysis of Water,"J. Electrochem. Soc. Vol. 122:739 (1975).
Single crystal TiO.sub.2 electrodes or doped single crystal TiO.sub.2 electrodes are often costly and difficult to produce. On the other hand, polycrystalline electrodes which utilize TiO.sub.2 as the photoactive semiconductor material are less difficult and less costly to produce, but are still limited in their spectral response to wavelengths of about 400 nanometers and shorter, corresponding to a band gap of approximately 3.0 eV or higher.
Another method of trying to alter the spectral response to TiO.sub.2 electrodes involves making physical mixtures of titanium dioxide and other compounds with optical absorptions closer to the desired optimum of the solar spectrum, see for example U.S. Pat. No. 4,181,593. The above-identified U.S. Patent teaches physical mixtures of titanium dioxide and other transition metal oxides which have a chemical oxidation state other than (+4) which are sintered and placed on a titanium metal substrate. While this teaching does produce an electrode, it does not produce an electrode which has the necessary efficiency to make it economically feasible in the market place. Additionally, the above-identified U.S. Patent teaches an optical absorption adjustment of only 70 nanometers at best, i.e., up to about 470 nanometers. This is still far from the optimum wavelength of approximately 800 nanometers. Yet another method used to modify TiO.sub.2 has been what is called "dying" of the TiO.sub.2 either supported by another substrate or unsupported. These systems use a film layering over the TiO.sub.2 of a material (frequently organic) which absorbs solar energy more efficiently than TiO.sub.2 alone. These systems, however, are deficient in a number of areas. First, they do not provide the longevity necessary for an economical system in the market place. Secondly, they are not efficient. A method similar to the "dying" method is that of layering TiO.sub.2 with a cover layer of one or more metal oxides which have a band gap more closely attuned to that of the solar spectrum. These systems, however, have all the limitations inherent in the "dying" type systems discussed above.
It is therefore an object of the present invention to provide a photoactive semiconductor film electrode comprising true solid/solid solutions of diverse metal oxides which are simple and inexpensive to produce; having a band gap and/or optical absorption optimized to the particular part of the energy spectrum of interest and which produce the necessary longevity. These and other advantages will become apparent in the following description of the instant invention.